1. Field of the Invention
The present invention relates to a semiconductor laser device, and a method for producing a compound semiconductor device such as the semiconductor laser device and a light emitting diode (LED). More particularly, the present invention relates to a semiconductor laser device having an excellent temperature property allowing continuous oscillation of visible light beams at room temperature, and to a method for producing a compound semiconductor device in which a semiconductor layer of a III-V group compound is formed on a GaAs substrate with high crystallinity.
2. Description of the Prior Art
In recent years, in order to attain high efficiency in optical information processing systems, a semiconductor laser device capable of oscillating light beams in the range of short wavelengths is required. Particularly, an (Al.sub.Y Ga.sub.1-Y).sub.0.5 In.sub.0.5 P crystal (0.ltoreq.Y.ltoreq.1) which lattice-matches a GaAs substrate has called attention in industries as a material for a visible light beam semiconductor laser capable of radiating light beams having a wavelength of the 600 nm band. The (Al.sub.Y Ga.sub.1-Y).sub.0.5 In.sub.0.5 P (0.ltoreq.Y.ltoreq.1) is hereinafter referred to as AlGaInP unless otherwise specified.
The molecular beam epitaxy (MBE), as well as the metal organic chemical vapor deposition (MOCVD), have been anticipated to be important methods for epitaxial growth of the AlGaInP crystal on the GaAs substrate. It has been reported that an AlGaInP group visible light semiconductor laser device produced by MBE method continuously oscillated visible light beams at room temperature. (Hayaka et al, Journal of Crystal Growth 95(1989) pp.949)
FIG. 8 is a sectional view of a conventional AlGaInP group visible light semiconductor laser device produced by the MBE method.
A first conductivity type GaAs buffer layer 72, a first conductivity type GaInP buffer layer 73, a first conductivity type AlGaInP clad layer 74, a GaInP active layer 75, a second conductivity type AlGaInP second clad layer 76 and a second conductivity type GaInP layer 90 are formed on a first conductivity type GaAs substrate 71 in such a manner that one layer is grown on another in this order by the MBE method.
On the second conductivity type GaInP layer 90 is formed an insulation silicon nitride film 91 which has a 10 .mu.m wide stripe groove extending to reach the second conductivity type GaInP layer 90.
Electrodes 85 and 84 are formed on the insulation silicon nitride film 91 and on the back surface of the substrate 71, respectively.
The semiconductor laser device shown in FIG. 8 is a gain guided semiconductor laser device in which current is confined by the insulation silicon nitride film 91 having the stripe groove. This semiconductor laser device has an oscillation threshold level of 93 mA and can continuously oscillate visible light beams at room temperature.
This type of the semiconductor laser device, however, can not effectively diffuse heat generated in the active layer at the time of oscillation due to the low thermal conductivity of the AlGaInP crystal. As a result, the maximum temperature for continuous oscillation is as low as 35.degree. C.
A semiconductor laser device having not only a structure of effective heat emission but also a double hetero structure composed of AlGaInP crystal layers lattice-matched with a GaAs substrate will be produced if an AlGaAs crystal layer with comparatively high thermal conductivity and effective heat emission can be formed on the AlGaInP crystal layers by the MBE method.
However, the AlGaAs crystal layer with high crystallinity may not be formed on the AlGaInP crystal layers lattice-matched with the GaAs substrate by the MBE method if the surface of the AlGaInP crystal layer is contaminated by impurities.
When the AlGaInP crystal layers and the AlGaAs crystal layer are continuously formed by the MBE method, the molecular beam radiation must be switched from P to As. The above contamination occurs at the time when the layer growth is temporarily stopped for this switching after the growth of the AlGaInP crystal layers is completed. In several seconds after stopping, impurities such as oxygen and steam in the atmosphere inside a MBE apparatus attach themselves to the surface of the crystal layer on which the growth is temporarily stopped.
Moreover, in order to grow the AlGaAs layer of high quality by the MBE method, the temperature of the substrate must be raised to about 620.degree. C. or higher. At such temperatures, In and P in the AlGaInP layers actively evaporate, causing to deteriorate the surface of the AlGaInP crystal layers. It is not possible to grow the AlGaAs crystal layer with high crystallinity on the deteriorated surface of the AlGaInP crystal layers.
Furthermore, the semiconductor laser device having the AlGaInP crystal layers grown by the MBE method is normally of a gain-guided type as shown in FIG. 8. In the gain-guided semiconductor laser device, the horizontal transverse mode of laser beams can not be fully controlled. Therefore, the development of an index-guided semiconductor laser device which can stabilize the horizontal transverse mode of laser beams is required also for the semiconductor laser device having AlGaInP crystals.
FIG. 9 is a sectional view of a conventional index-guided semiconductor laser device. A first conductivity type GaAs buffer layer 72, a first conductivity type AlGaInP first clad layer 74, a GaInP active layer 75, a second conductivity type AlGaInP second clad layer 76, a second conductivity type GaAs layer 78 and a second conductivity type InGaAs layer 100 are formed on a first conductivity type GaAs substrate 71 in such a manner that one layer is grown on another in this order by the MBE method.
The GaInP active layer 75, the second conductivity type AlGaInP second clad layer 76, the second conductivity type GaAs layer 78 and the second conductivity type InGaAs layer 100 are etched to form a 10 .mu.m wide ridge. This ridged surface is covered with a silicon oxide layer 101 except the top portion thereof. Electrodes 85 and 84 are then formed over the top ridged surface and on the back surface of the substrate 71, respectively.
In the semiconductor laser device of the above described structure, current flows between the electrodes 85 and 84 through the top portion of the ridge where the silicon oxide layer 101 is not formed. And, the presence of the 10 .mu.m wide thin active layer 75 enables the oscillation at a unified horizontal transverse mode.
However, the semiconductor laser device of this structure is disadvantageous in that heat generated in the active layer 75 is not efficiently emitted outside the device due to the presence of recesses on the ridged surface formed by etching, as a result preventing continuous oscillation at room temperature.